For close to 58 years, Mount
Vesuvius has sat relatively quiet above the city of Naples, Italy. The active
volcano trembles every now and then with small earthquakes and may even vent
low-temperature gases, but Vesuvius has not erupted since World War II. Volcanologists
expect that its next eruption will be explosive, although they cannot predict
when the event will occur.

Now, some Italian
and French researchers say that a massive magma layer beneath Mount Vesuvius
may hold the key to understanding the historic volcano’s past and future. Reporting
in the Nov. 16 Science, Emmanuel Auger and colleagues used seismic data
to locate a magma reservoir 400 square kilometers wide that sits 8 kilometers
below Mount Vesuvius.

[ At right, Mount Vesuvius as seen from
the city of Naples, Italy. Mt Vesuvius is perhaps best known for burying the
Roman towns of Pompeii and Herculaneum with hot lava flows and debris in A.D.
79. The volcano also erupted in 472 and 1631. More frequent, less explosive
eruptions occurred between 1631 and 1944. Courtesy of Paul Gasparini.]

Researchers were surprised to find such a large amount of magma under Vesuvius,
says Auger, a researcher at the Università di Napoli Federico II in Naples.
“We believe it is a regional item and that it is probably present beneath the
nearby Phlegrean Fields volcano. No estimation of the magma layer’s extent had
been made before.”

Locating the reservoir does not help scientists predict the volcano’s next eruption,
but it does identify areas that could provide seismic clues. “If deep seismic
events were detected at 6 to 8 kilometers, it might mean that magma is rising,”
Auger says. By creating a clearer picture of the way the magma below Mount Vesuvius
interacts with the crust, researchers can better understand the volcano’s pattern
of expected activity and ground deformation.

Using seismic tomography, Auger’s team probed beneath Mount Vesuvius, gaining
glimpses of the crust’s structure. They created explosions to generate seismic
waves, followed the waves’ movements through the crust and then gathered data
on the speed and direction of the waves arriving at 25 stations. To create the
explosions, Auger’s team used 1,800 shots from 16-liter air guns on a ship in
the Bay of Naples.

The team estimated S-wave velocities between 0.6 and 1 kilometer per second
and P-wave velocities of approximately 2 kilometers per second. “I think the
most important finding is the quantitative estimation of the S wave velocity
in the low velocity layer: the very low value is a very strong indication of
the presence of a semi-molten material. We can now say it is magma,” Auger says.

Past research hinted at the existence of such a magma reservoir. In 1994, Aldo
Zollo and others detected an area of low seismic wave velocity within the crust
below Mount Vesuvius. Seismic waves travel slower through a liquid than a solid,
and wave changes most often occur at a boundary between two different geologic
layers. Therefore, the conversion and slowing down of the waves implied the
possible presence of a deep melting zone.

This latest research strengthens those past results and adds a new layer to
the model of Mount Vesuvius’ magma feeding system, Auger says. “The scenario
for the Vesuvius magma feeding system now has two magma reservoirs on which
all geologists agree: a large, deep one, the magma layer; and a shallower, smaller
one,” Auger says. The tomographic images indicate that the magma lies flat and
parallel to the crust’s layers. Denser and more buoyant than the surrounding
rock, the magma rises toward the crust and reaches neutral buoyancy with the
rock, floating in a trapped layer.

Auger says that isotopic studies of the material ejected during the strongest
eruptions also support their model. Isotopic ratios in Neapolitan volcanic magma
show signs of significant mixing with the surrounding rock, suggesting the reservoir
might look more like a sponge, with the magma seeping through fractures in the
rock. The magma has a two-fold origin, Auger says, where the 400-square-kilometer
magma layer feeds into several smaller layers closer to the surface.

Another volcanologist, Tobias Fischer of the University of New Mexico, says
that the resolution of the study does not elucidate whether the magma is completely
molten or partially molten. Still, Fischer adds, a partially molten body of
magma in a dendritic fracture system of rock seems to be a plausible interpretation
of what is happening below Vesuvius. He adds that Auger’s observations are consistent
with and supportive of the idea that magma below Vesuvius remains at mid-crustal
levels for a long time and interacts with the crust.

“In terms of forecasting a future eruption, I do not think that the findings
can be directly used to forecast the next eruption. The findings, however,
are critical for the interpretation of seismicity and gas emission data that
may lead up to the next eruption because they constrain the depth and extent
of the magma body below the volcano,” Fischer says.